Golf balls, whether of solid or wound construction, generally include a core and at least a cover and/or outer coating. The core may be solid or liquid-filled, and may comprise one piece or have a center with one or more outer core layers formed about the center. Covers may also be formed of one or more layers. Multi-layer cores and covers are sometimes known as “dual core” and “dual cover” golf balls, respectively.
The playing characteristics of golf balls, such as spin, feel, CoR and compression can be tailored by varying the properties of the golf ball materials and/or adding additional golf ball layers such as at least one intermediate layer disposed between the cover and the core. Intermediate layers can be of solid construction or may be formed of a tensioned elastomeric winding, which are referred to as wound balls. The difference in play characteristics resulting from these different types of constructions can be quite significant.
Cores are generally made using techniques such as compression or injection molding. Typically, the center is formed by compression molding a slug of uncured core material into a spherical structure. The outer core layers may be formed, for example, by molding compositions over the center by compression or injection molding techniques. In turn, the intermediate and/or cover layers are applied.
A cover layer(s) may be formed over the outermost of the core or intermediate layer (collectively referred to herein as “ball subassembly”) using suitable techniques including, for example, compression-molding, flip-molding, injection-molding, retractable pin injection-molding, reaction injection-molding (RIM), liquid injection-molding, casting, spraying, powder-coating, vacuum-forming, flow-coating, dipping, spin-coating, and the like. In a compression molding process, hemispherical shells are generally placed about the subassembly in a compression mold and fused together under sufficient heat and pressure. In contrast, with an injection molding process, cover material is injected about and directly onto the subassembly using retractable pins, for example.
When a cover layer is formed by a casting process, liquid cover material is poured into lower and upper mold cavities, into which a subassembly is lowered at a controlled speed. The subassembly is held in place via partial vacuum to the point of sufficient gelling, and then the upper mold cavity is mated with the lower mold cavity under sufficient pressure and heat followed by cooling the unit until it can be handled without deformation.
Golf ball core and cover layers are typically constructed with polymer compositions such as polybutadiene rubber, polyurethanes, polyamides, ionomers, and blends thereof. Ionomers, particularly ethylene-based ionomers, are a desirable group of polymers for golf ball layers because of their toughness, durability, and wide range of hardness values. Further, golf balls incorporating fatty acid neutralized acid polymers are generally known for achieving desirable golf ball properties relating for example to spin, feel, and CoR.
In this regard, fatty acid neutralized acid polymers may be manufactured by feeding acid copolymers and organic acids into a melt extruder such as a single or twin screw extruder via separate feed lines, and adding a suitable amount of cation source for neutralizing a desired level of acid groups present. The ingredients may be intensively mixed prior to being extruded as a strand from the die-head.
Several drawbacks have presented from feeding each ingredient into the extruder individually. First, formulation errors occur because coordinating the varied feed rates of multiple feed lines can be difficult. Additionally, the organic acid sometimes fails to distribute uniformly throughout the mixture within the extruder in relation to the acid copolymer, thereby producing a resulting material lacking homogeneity in localized areas—a quality control issue which increases the overall cost of golf ball manufacture.
Accordingly, there remains a need for improved methods for manufacturing neutralized polymers suitable for golf ball constructions as well as to methods for making golf balls incorporating neutralized polymers and to golf balls incorporating such neutralized polymers which reduce the margin for formulation error and lower manufacturing costs. The present invention addresses and solves this need.